Recently, with the consideration of costs, in some AC-to-DC power converters, a BJT is used instead of a metal-oxide-semiconductor field-effect transistor (MOSFET) as the power switch, such as those described in U.S. Pat. Nos. 8,045,348 and 7,961,484 and U.S. Pat. Application Publication No. 2010/0202165.
BJT driving technique includes two types, namely base driving (BD) and emitter driving (ED). FIG. 1 shows an AC-to-DC power converter using BD technique, in which a rectifier circuit 10 rectifies an alternating current (AC) voltage VAC to generate an input voltage Vin, in a BJT Q1, a collector works as an input terminal connected to an inductor Np, and an emitter works as an output terminal connected to sensing resistor Rcs, and a control integrated circuit (IC) 12 has a pin Output connected to a base of the BJT Q1 and providing a base current Ib to switch the BJT Q1 to convert the input voltage Vin into an output voltage Vo. The control IC 12 further has a pin Isense connected to a sensing resistor Rcs for detecting an output current Ie of the BJT Q1. When the BJT Q1 is on, the inductor current Ic of the inductor Np raises. Ideally, the inductor current Ic is close to the current Ie, so that the control IC 12 can determine the inductor current Ic according to the voltage Vcs at the sensing resistor Rcs, and turn off the BJT Q1 when the inductor current Ic reaches a predetermined peak. In addition, the internal circuit of the control IC 12 can also set the base current Ib according to the voltage Vcs. However, the control IC 12 can only provide the base current Ib in a fixed range. Once the required base current Ib is not within the fixed range, the optimal performance becomes unachievable.
FIG. 2 shows an AC-to-DC power converter using another BD technique, in which a rectifier circuit 10 rectifies an AC voltage VAC to generate an input voltage Vin, a control IC 14 has a pin Base connected to the base of a BJT Q1 to switch the BJT Q1 and thereby convert the input voltage Vin into an output voltage Vo, and a current-setting resistor Rset is connected between pins VDD and VDD-B. The current-setting resistor Rset generates a base current Ib for the control IC 14 according to a voltage drop therein. FIG. 3 shows the control IC 14 as shown in FIG. 2, which includes switches SW1 and SW2 and a driver 16. When the driver 16 turns on the switch SW1 and turns off the switch SW2, as shown at the time t1 in FIG. 4, the current-setting resistor Rset generates a base current Ib for the base of the BJT Q1 to turn on the BJT Q1. The increase of the inductor current Ic induces the increase of the voltage Vcs of the sensing resistor Rcs. When the voltage Vcs reaches a predetermined threshold, the driver 16 turns off the switch SW1 and turns on the switch SW2 to turn off the BJT Q1, as shown at the time t2 in FIG. 4. In this AC-to-DC power converter, the base current Ib of the BJT Q1 is determined by the current-setting resistor Rset outside the control IC 14, so a user may select an appropriate current-setting resistor Rset for the base current Ib as desired. However, an additional pin VDD-B is needed for this purpose. Furthermore, one characteristic of BJT is that when the BJT Q1 is turned off, the output current Ie of the BJT Q1 ends, but BJT Q1 will generate a recovery current Irb that flow to the base of the BJT Q1 from its collector. This causes the inductor current Ic of the collector of the BJT Q1 remains going up for a period of time after the current Ie ends, as happening during the tome period between time points t2 and t3 in FIG. 4, making the voltage Vcs of the sensing resistor Rcs unable to represent the inductor current Ic, and hindering the control IC 14 from getting the correct inductor current Ic by referring to the voltage Vcs.
FIG. 5 shows an AC-to-DC power converter using ED technique, in which a rectifier circuit 10 rectifies an AC voltage VAC to generate an input voltage Vin, a control IC 18 has a pin ED connected to the emitter of a BJT Q1 to switch the BJT Q1 and thereby convert the input voltage Vin into an output voltage Vo, a current-setting resistor Rset has one terminal connected to the base of the BJT Q1, and another terminal connected to the base of a BJT Q2, and thus, by selecting the current-setting resistor Rset, the base current Ib may be obtained as desired. The control IC 18 has a pin GND connected to a first terminal 20 of the sensing resistor Rcs and determining a reference potential for the control IC 18 according to the voltage Vgnd thereon, and has another pin CS connected to a second terminal 22 of the sensing resistor Rcs, so that the control IC 18 can determine the inductor current Ic of an inductor Np according to the voltage drop Vcs−Vgnd in the sensing resistor Rcs. FIG. 6 shows the control IC 18 shown in FIG. 5, which includes a switch SW3 connected between pins ED and GND, and a driver 24 for controlling the switch SW3. When the driver 24 turns on switch SW3, the current-setting resistor Rset generates the base current Ib according to the base voltage V2 to turn on the BJT Q1, as shown at time t1 in FIG. 7. The inductor current Ic at the collector of the BJT Q1 and the output current Ie of the BJT Q1 accordingly raise. At this time, the current Ie passes through the sensing resistor Rcs. Since the voltage Vgnd at the first terminal 20 of the sensing resistor Rcs is the reference potential for the control IC 18, from the perspective of the control IC 18, the voltage Vcs−Vgnd=−Ie×Rcs at the pin CS has a negative value, as shown by the waveform of Vcs−Vgnd shown in FIG. 7. When the voltage Vcs−Vgnd reaches the predetermined threshold, the driver 24 turns off the switch SW3 to turn off the BJT Q1, as shown at time t2 in FIG. 7. However, as stated previously, when the BJT Q1 is turned off, the BJT Q1 will generate the recovery current Irb that makes the voltage drop Vcs−Vgnd of the sensing resistor Rcs unable to represent the inductor current Ic, as happening during the time period between the time points t2 and t3 in FIG. 7. This hinders the control IC 18 from getting correct inductor current Ic by referring to the voltage drop Vcs−Vgnd of the sensing resistor Rcs.
Additionally, each of the AC-to-DC power converters shown in FIG. 1, FIG. 2 and FIG. 5 determines the inductor current Ic according to the voltage Vcs of the sensing resistor Rcs, and turns off the BJT Q1 when the inductor current Ic reaches the predetermined peak Ipeak. However, between the time that the inductor current Ic reaches time predetermined peak Ipeak and the time that the BJT Q1 is turned off, a delay time Td is caused by the charge stored when the BJT was on, as shown in FIG. 8. The delay time Td makes the peak of the inductor current Ic goes beyond the predetermined peak Ipeak. Moreover, the slop of the inductor current Ic varies with the input voltage Vin. When the input voltage Vin is low, the slops of the inductor current Ic is relatively small, and the peak error ΔI1 is relatively small, as shown by the waveform 26 in FIG. 8. When the input voltage Vin is high, the slope of the inductor current Ic is relatively large, and the peak error ΔI2 is relatively large, as shown by the waveform 28 in FIG. 8.